This invention relates to an electrical generator drive having a power source which is liable to transmit power at a rotational speed which may fluctuate substantially in service, an electrical generator which requires a rotational input of power at a substantially constant speed, and a controllable drive transmission of high rotational inertia coupling together the power source and the electrical generator.
1. Field of the Invention
The invention is particularly concerned with an electrical generator device which derives power from a power source which provides a speed output which can vary quite substantially in service, particularly the main engine on a marine vessel driving the propeller, though other examples of fluctuating power source with which the invention may be used include wind or water-powered devices. On a marine vessel, it is usual to employ an A.C. generator, but if the input speed to the generator varies, the frequency of the current produced will also vary, causing problems with some electrical equipment. For example, clocks will not keep correct time, and AC motors will run at variable speeds. Similarly, if a DC generator is used, the voltage of the current produced will also vary.
The generation of electrical power on board a ship can be derived from the prime mover of the ship, when it is not in port, since this is more economical than to operate a secondary auxiliary engine. However, a disadvantage of using the prime mover is that its speed, and that of the drive train, from the primer mover to an AC generator, may vary depending upon external factors e.g, if the ship is sailing through a storm. When a ship is cruising at a nominally constant speed, it might be expected that the engine(s) and propeller(s) would be rotating at a constant speed, but this is not always the case. Factors such as the roll of the ship, wave motion, changes in local water temperature and density, for example, can lead to a surprisingly large and random variation in the propeller speed. Thus, if a powered generator is driven via the prime mover drive train, any variation of the speed of rotation of the propeller shaft will be magnified by the gear ratio of a take-off drive to the generator, so that the rotational speed of the generator will vary through a greater magnitude speed range. In modern ships, alternating current power is generally used so that, if the speed of the AC generator varies, the current produced will have a variable frequency.
2. Description of the Prior Art
It has become common practice to electronically rectify the variable frequency to stabilize the frequency of the variable current produced from shaft driven generators, but for a large ship with a power requirements of between 0.5 and 1 MW, the physical size of the electronic equipment and the associated power loss pose severe problems for ship designers and operators.
Also, the generation of electrical power from natural sources, such as air or wind-driven devices, or water driven devices, e.g. turbines driven by tidal or other currents and devices arranged to extract energy from wave motion, usually suffers from the common disadvantage of fluctuation in the output speed of the device.
It is known from U.S. Pat. No. 3,298,251 to provide an electrical generator drive which is intended specifically for use with a turbo-jet engine. However, in this case the power source (the turbo-jet engine) will normally provide input power at a substantially constant speed, or at least the input speed to the generator drive will vary only gradually following operation of the throttle of the engine. Further, a hydraulic governor unit is provided in order to maintain a substantially constant input speed to the generator, and a governor unit of this type is particularly suitable for following progressive speed changes which would occur in service with a turbo-jet engine.
By contrast, the invention is concerned with providing an electrical generator drive which derives power from a power source which provides a rotational input of power to the generator which may fluctuate quite substantially in service, and any changes in speed are step changes, rather than progressive alterations in speed. Furthermore, the controllable drive transmission in an electrical generator drive according to the invention has a very high rotational inertia, and this magnifies the adverse effects of step changes in input speed to the generator drive from the power source.